1. Field of the Invention
The present invention relates to electric plug type fuses. More specifically, it relates to a construction which assures a high level of structural and electrical integrity of plug type fuses, while permitting use of fusible elements substantially thinner than heretofore used.
2. Description of the Prior Art
Electric plug type fuses typically include a cylindrical hollow glass body which defines an internal cavity and which are provided with screw threads on the outer surface thereof. A contact button support made from an electric insulating material has one end projecting into the internal cavity of the glass body and the other end extends to the outside thereof. A contact button is carried on the outer end of the contact button support. A metal outer shell overlaps the contact button support and is provided with screw threads which mate with the screw threads on the glass body thereby allowing the metal shell to be threadably engaged with the glass body and to retain the contact button support in position with respect to the glass body. The glass body is provided with a groove on the outer surface which extends perpendicular to and interrupts the screw threads formed on the glass body. This groove and the outer metal shell cooperate to define a channel between them. The fusible element of such plug fuses is formed from a strip like conductive material and has one end thereof conductively connected to the contact button inside the fuse and extends from that connection into the cavity defined by the glass body. From there the fusible element extends in between the contact support and the glass body and into the channel defined by the groove and the metal shell. The end of the fusible element extending into this groove is soldered to the metal shell to complete the electrical path of the fuse.
With prior art fuse designs, the fusible element was relatively thick, for example in the neighborhood of twelve thousandths of an inch thick. This relatively thick element provided a good mechanical interlock between the glass body and the outer metal shell when the element was fitted into the groove there between and soldered to the outer shell. More recent designs of plug type fuses incorporate much thinner element material, for example, five thousandths of an inch thick is not uncommon. Thinner elements do not provide as strong of an interlock between the hollow glass body and the outer metal shell as designs using thicker elements. This has resulted in a number of problems with use of thin elements in such fuses.
Electric plug fuses are installed into a fuse holder by screw threading the threads on the metal shell into engagement with mating metal threads in an appropriate fuse holder. During installation the glass body is held by the installer in order to impart the necessary rotation to the fuse to effect installation. During such installation the clockwise force imparted on the glass body is transferred to the metal shell by way of the groove in the glass body, the fusible element contained in the groove and the solder connection interconnecting the fusible element with the metal shell. Installation of a new fuse in a fuse holder normally does not exert a great force on this interconnection.
When an electric plug fuse has blown and it is necessary to remove the fuse from the fuse holder, however, the force necessary to remove the fuse is usually far greater than that necessary for installation of the fuse. Such is the case in particular when a fuse has been installed in a holder for an extended period of time. During removal of a blown fuse, the counterclockwise rotational force imparted on the glass body is again transferred to the metal shell by way of the mechanical interconnection between the groove in the glass body, the fusible element, and the solder joint between the fusible element and the screw shell. Under such circumstances, however, the resistance of the metal shell to rotation often results in a force upon the above described interconnection which results in failure of the connection. Such failure results in the fusible element passing out of the groove and into the region between the threaded glass body and the metal shell. In designs using a thin fusible element, it has happened that when removing a fuse from its fuse holder, the inteconnection has failed completely and the glass body has been turned out of the metal screw shell leaving the remaining element, the insulating tip and the center contact in the fuse holder, thus resulting in an extremely hazardous condition.